Tanning with lignin sulfonic acid-chromium compound reaction product



5 415 TANNING WITH SULFONIC ACID-CHRO- Ohio, a corporation No Drawing.Filed May 4, 1960, Ser. No. 26,675 12 Claims. (Cl. 8-94.26)

This invention relates to a new and improved process for the tanning ofleather and more particularly relates to a new and improved leathertanning process which. utihas a chromium-containing tanning composition.

This application is a continuation-in-part of our cc- 1957, nowabandoned.

At the present time, two recognized types of tanning procedures havebeen accepted and widely-practiced in tages which hasjustified itscontinued use despite other somewhat serious disadvantages, which at thepresent time, have not been completely overcome. For example, vegetabletanning produces leather and particularly sole leather of an excellentcolor, having wear properties and waterproofness which are generallyquite satisfactory. However, vegetable tanning is an extremely slowprocess and hence, despite the advantage of producing a so-called fullleather of good color, the slowness of vegetable tanning has precludedits use in many applications where it otherwise would be highlydesirable.

0n the other hand, the outstanding advantage of chrome tanning is thespeed with which the tanning is effected. Moreover, chrome tannedleather exhibits good wear and abrasion resistant properties. However,the principal disadvantages of conventionally chrome tanned leather,especially when used as sole leather, are that it is susceptible todistortion and at times is slippery on wet surfaces.

In an effort to overcome the recognized disadvantages of vegetable andchrome tanning, prior workers in the art have proposed severalvariations in tanning procedure. For example, it has been suggested thatleather first be tanned with a vegetable tanning material and thenretanned with a cationic chromium-containing tanning composition, i.e.,the conventional type of chrome tanning composition. Advantages of sucha procedure are that a higher shrinkage temperature is obtained,together with increased abrasion resistance, especially in sole leatherso prepared. However, it has been found that leather thusproducedfrequently has a poor color. Moreover, since such a procedure involvesadditional treatment, inevitably there is-an increase in cost involved.In addition, the extremely slow vegetable tanning process, used as afirst step, still is encountered.

' Alternatively, prior workers in the art also have proposed first tochrome tan leather and then to use a vegetable tanning composition as aretan. However, this technique has not been found to be the completeanswer to the problem either, because a heavy vegetable tan frequentlyis required to obtain a satisfactory leather 'color. Inasmuch, as thedesired wear properties of the leather generally decrease with anincrease in the amount of vegetable tanning effected. this procedure hasresulted in a leather, and particularly sole leather, having poor UnitedStates Patent 0 pending application Serial No. 650,825, filed April 5,

2,995,415 Patented Aug. 8, 1961 ice 2 Accordingly, itwillbeappreciatedthatup to the present time no completely satisfactory tanning techniquehas been available for the tanning of leather, particularly ,leatherdestined for application as sole leather.

It is, therefore, the principal object of the present invention toprovide an improved leather, especially sole leather, characterized by amaximum of the desirable properties of vegetable-tanned leather as wellas those of chrome-tanned leather, while minimizing the disadvantages ofboth tanning techniques.

A further object of the present invention is to provide a new andimproved tanning process by means of which this improved leather can beproduced.

These and other objects of the present invention will become apparent'tothose skilled in the art from the description of the invention whichfollows.

The present invention envisions a tanning process which utilizes apredominantly non-ionic, chromium-containing composition to produce aleather characterized by good color, solidity, fullness, workability,and excellent abrasion and water resistance. Such tanning composition isobtained by chemically reacting lignin sulfonic acid with at leastsufiicient of a hexavalent chromium compound to produce a predominantlynon-ionic, chromium-containing material.

In preparing this non-ionic, chromium-containing composition, the amountof hexavalent chromium compound employed must not be less than about 25%by weight of the lignin sulfonic acid, expressed in terms of ligninsolids used in preparing the lignin sulfonic acid. Moreover, it has beenfound that the amount of hexavalent chromium compound employed shouldnot exceed about by weight of the lignin sulfonic acid, again expressedin terms of lignin solids. It will, of course, be understood that thehexavalent chromium compound may be chromic acid or an alkali metalbichromate, the term alkali me referring to sodium, potassium, lithium,rubidium and cesium. However, because of its low cost and readyavailability, the preferred hexavalent chromium compounds are the alkalimetal bichromates, of which sodium bichromate is the most preferred. Forthis reason, hereinafter particular reference will be made to sodiumbichromate as the source of hexavalent chromium.

The expression lignin sulfonic aci as used throughout the specificationand claims is intended to refer to lignin sulfonic acid obtained fromneutralized sulfite waste liquors. In practice, these liquors, formed asa by product from the cooking of wood fibers in the paper industry, aregenerally neutralized with calcium or magnesium salts and the resultingliquors used as such or spray dried to obtain a crude water-solublelignin sulfonate, generally calcium sulfcnates, or as termed herein,lignin solids. One commercially available product of this type is termedGoulac." The water-soluble solids can be dissolved in water andacidified prior to use in the practice of this invention, or, ifdesired, may be formed in situ during the preparation of the compositionembodying the present invention.

A typical commercially available Goulac (lignin solids) has thefollowing analysis:

The term non-ionic as applied to chromium throughout the specificationand claims is intended to refer to chromium, the major part of which hassubstantially no electrical charge as detectable by ion exchangetechniques. Thus, the chromium-containing taming composition used in thepresent method comprises chromium in a non-ionic form as a majoringredient, i.e., an amount of at least 50% by weight. Attention is nowdirected more specifically to the various aspects of the invention,notably the tanning composition, its preparation and use in tanning.

The tanning composition of this invention comprises the chemicalreaction product of a major proportion of lignin sulfonic acid, e.g.,prepared by the acidification of dissolved lignin solids, with a minorproportion of a hexavalent chromium compound, e.g., an alkali metalbichromate, preferably sodium bichromate, in the amount of at least 25%by weight of the lignin solids, which amount is suflicient to produce apredominantly nonionic, chromium-containing reaction product in whichthe content of non-ionic chromium is at least 50% by weight. Thiscomposition is desirably partially neutralized to an acid pH within therange from about 3.0 to 4.5, 4.0 being preferred.

The proportion of the reactants which are employed is critical, in thatthe minimum amount of the hexavalent chromium compound, especiallysodium bichromate, which is to be used, must be at least 25% by weightof the lignin solids, which amount is suflicient so that the reduciblehexavalent chromium present is converted in the reaction product to apredominantly non-ionic form in the amount of at least 50% by weight.Moreover, the maximum amount of hexavalent chromium compound to beemployed is found to be about 50% by weight of the lignin solidsinasmuch as when greater amounts are used, the product does not containa predominant proportion of non-ionic chromium. Additionally, if anamount of sodium bichromate greater than 50% by weight of the ligninsolids is used, the reaction mixture gels and becomes virtuallyimpossible to work with.

Preferably, the reaction product is partially neutralized to reduce itsotherwise extremely low pH to a higher, but still acid pH of about 3.0to 4.5, as pointed out hereinabove. The reaction product may be used assuch, without further treatment, as a solution or may be concentrated ordried. Preferably, the reaction product is spray dried at an elevatedtemperature.

In practice, the desired amount of water-soluble lignin solids isdissolved in water, with agitation, and the resultant solution isacidified using a mineral acid, such as hydrochloric or sulfuric acid,the latter of which is preferred. The amount of sulfuric acid used ispreferably about 35% by weight of the lignin solids. Alternatively to insitu formation of the lignin sulfonic acid, an aqueous solution oflignin sulfonic acid may be employed. In either case, to the resultantlignin sulfonic acid solution with continuous agitation, is added anaqueous solution of a hexavalent chromium compound. Preferably an alkalimetal bichromate and especially sodium bichromate, typically as a 50% byweight solution. The rate of addition of this solution is such that incombination with the agitation, the formation of lumps or gel particlesis avoided. Typically, in a commerical operation, the addition time isat least one hour, 1-2 hours being preferred. This solution is added inthe amount of at least 25 by weight as sodium bichromate, of the ligninsolids, and not in excess of 50% by weight as sodium bichromate, of thelignin solids. Continuous agitation is effected until the chemicalreaction and dissolution are complete.

The resultant solution, generally having a pH of about 2, may be used assuch in'those applications wherein extremely acid material is notobjectionable. However, it generally is desirable and is preferred,partially to neutralize this solution by addition of alkali, e.g.,sodium hydroxide, sodium bicarbonate or other alkaline substance, toraise the pH to a value of about 3.0 to 4.5, preferably about 4.0.Generally, the sodium hydroxide is added in an amount of about 11% by.weight of the lignin solids used.

The partially neutralized solution then may be used as such or,depending upon the application, may be concentrated or dried. Anespecially advantageous treatment constitutes spray drying of thematerial at an elevated temperature. Typical spray drying conditionsinvolve adjusting the desiccating chamber of the drier so as to providean air inlet temperature of about 300 to 500 F., preferably about 400F., and an air outlet temperature of about 200 to 250 F., preferably 210to 230 F.

The practice of the present invention in respect to tanning of leatherinvolves subjecting the hides to be tanned, after conventional soakingor washing, fleshing, dehairing, bating and pickling, to an aqueoussolution of a predominantly non-ionic, chromium-containing reactionproduct obtained by chemically reacting lignin sulfonic acid with thehexavalent chromium compound in the amount of at least 25% by weight ofthe lignin solids in the lignin sulfonic acid, at an elevatedtemperature for a period of time sufiicient to effect the desiredtanning. In many instances, it is advantageous to soak the stockimmediately prior to tanning, in an aqueous solution of an alkali metalhalide, notably sodium chloride, e.g., in a l-l5% by weight sodiumchloride solution.

In the carrying out of tanning in accordance with this invention, it isdesirable to maintain the pH of the tanning solution within the range ofabout 4.0 to 6.0, 5.0 being the specifically preferred pH. Additionally,tanning is preferably carried out attemperatures above ordinary roomtemperature, e.g., from about to F. and preferably about 100 F.,although it will be appreciated that in some applications higher tanningtemperatures, i.e., to about F. may be used, the upper temperature limitgenerally being dictated by the quality of leather which is required.Conversely, a minimum practicable tanning solution temperature generallyis about 85 F. but is, of course, dictated by the time available fortanning.

The time required for tanning in accordance with the practice of thepresent invention depends on a large number of variables, including thetype of stock, thickness, prior treatments, and effect desired, so thata satisfactory shrink temperature can be obtained. It is understood thatby shrink temperature is meant the elevated temperature at whichshrinkage of the leather in water, or a glycerine-water mixture, firstis observed, which temperature is used as a measure of tanningeffectiveness, a shrink temperature of 200 F. or higher indicating asatisfactorily tanned leather. As a general guide, however, it isdesirable to subject the hides to the tanning solution of the presentinvention for a period of at least 1 hour, typically l-10 hours andpreferably about 8 hours at the temperatures indicated hereinabove. Inthis manner, shrink temperatures of 200 F. or higher are readilyobtained.

The concentration of the tanning solutions may also be varied, itgenerally being desired to use a tanning solution containing about20-35% solids, 30% solids being especially preferred, based on theweight of the wet hides to be tanned. Moreover, the amount of thetanning solution used is generally about 60-120% by weight of the wethides to be tanned. Following this tanning procedure, the thus-tannedhide may, if desired, advantageously be retained with a vegetabletanning material, a conventional chrome-tanning material or, with thepredominantly nonionic tanning composition of the present invention or acombination of chrome and vegetable retans. Alternatively, the leathermay be used without further treatment or, it may, be fat liquored and/orwaterproofed prior to usage with or without any of the aforementionedretannings.

- In order that those skilled in the art may better under stand themethod of the present invention and the manner in which it may bepracticed, the following specific examples are given. It is to beunderstood, of course, that the examples are not to be constituted aslimiting the invention but are intended merely to be illustrativethereof.

EIAMPLEI One hundred lbs. of Goulac (lignin solids) is dissolved in 250lbs. (30 gallons) of water and 35 lbs. of 95% by weight sulfuric acid isadded thereto and the resulting mass is agitated for 1 hour, duringwhich time SO, fumes are evolved. Twenty-five lbs. of Na=Cr O-;.2H= as a50% by weight solution is then added slowly with continuous and vigorousagitation to avoid formation of any chrome-lignin gel lumps. After allthe bichromate solution is added, the mass is agitated for 2 hours afterwhich time 5.5 lbs. of sodium'hydroxide is added, as a 25% solution,with continuing vigorous agitation. The resultant mixture is allowed tostand overnight, after which time another 5.5 lbs. of sodium hydroxide,as a 25% solution, is added with vigorous agitation and suflicient wateris then added to make a total volume of 50 gallons. The pH of theresultant liquid tanning solution is 4.0. Chemical analysis of theliquid tan so prepared is as follows:

Percent by weight Solids 28.6 C130, 2.52 Anionic chromium 23.9 Non-ionicchromium 70.7 Cationic chr m 5.4

This material can be employed either as a liquid or spray dried,advantageously with the desiccating chamber of the dryer adjusted so asto provide an inlet air temperature of about 400 F. and an outlet airtemperature of about 210 to 230 F., during the drying operation.

EXAMPLE 2 Stock, comsisting of pickled steer hides, is floated in atanning drum in a 10% sodium chloride solution for a period of '10minutes, the amount of sodium chloride solution being equal to theweight of the stock. To the thus-treated stock is added the liquid tanprepared in Example 1 in the amount of 50% by weight of the stock.Tanning is carried out for 2 hours at 100 F. after which time anadditional quantity of the liquid tan in the amount of 50% by weight ofthe stock is added and tanning is continued for a total of 4 hours.Thereafter, a 5% aqueous solution of sodium bicarbonate is added to thetanning drum in the amount of 3% by weight of the stock so as to raisethe pH of the tanning solution to 5.0. Tanning is continued for a totaltanning time of 8 hours, after which time a shrink temperature of 200 F.is obtained. The stock is then horsed up overnight and washed with wateruntil a clear eflluent is obtained. The thus-tanned leather may beemployed as such or may be subjected to further treatment as indicatedin the following examples.

EXAMPLE 3 cno, n.0, Basiclty Percent Percent Percent 23.5 1.0 ss-ssprocedure, an excellent leather is obtained.

EXAMPLE 4 Stock produced in accordance with Example 2. is retanned witha vegetable tan. 2 to 4% of a fat liquor is added to the vegetable tanafter .6 hour of the tanning run. Using this procedure an excellentleather is obtained.

' EXAMPLE 5 The leather produced in accordance with Examples 2 and 3 isfat liquored using a 2% solution of Ncutrolene 7A (a cationic-non-ionicanimal type fat liquor) in an amount equal to the weight of the stock tobe fat liquored, at F. Using this procedure, a complete take up of thefat liquor by the stock is elfected. Advantageous fat liquoring can alsobe effected using a fat liquor comprising raw neats-foot oil and anemulsifier therefor such as the various cationic emulsifying materials.

Leather treated in accordance with the present invention, if desired,may be rendered even more water-repellant by further treatment with awater repellant, such as silicone resins, e.g., Dow Corning Silicone1109 or Qgilon (stearato chrome complex in isopropyl alcohol EXAMPLE 6To illustrate the excellent properties of leather obtained by thepractice of the present invention, leather tanned in accordance with theprocedure set forth in Example 2 is subjected to water absorption testsin which a weighed specimen 0f leather is immersed in distilled waterfor periods of 30 minutes and 120 minutes, any gain in weight beingcalculated as a percentage gain of the conditioned weight of the sample.Additionally, the leather is subjected to an abrasion test usinga US.Bureau of Standards abrasion testing machine in which the wear loss inthickness is measured against a standard sole leather and the resultsreported in the number of cycles necessary to wear away 0.05 inch and0.10 inch using 1 x 1 inch test pieces with 3M No. 2 /2 garnet paper. Inthese tests, leather tanned using the chromium tanning compositionprepared in accordance with Example 1 is compared to leather tanned witha composition prepared in the same manner with the exception that theamount of sodium bichromate used is only 20% by weight of the ligninsolids rather than 25% by weight. Additionally, two tests are made usingleather tanned with a tanning composition prepared by reacting thelignin sulfonic acid with 35% and 40% sodium bichromate by weight of thelignin solids. The results of these tests are as follows:

Sodium 181- Abrasion Cycles Percent Water chromate/ Absorption Test No.Llg'nln Solids Ratio 0.05 In. 0.10 In. 0.5 hours 2.0 hours 0. 25 1, 7002, 617 6 I 0. 20 2,017 3, 033 21. 6 31. 8 0. 25 2, 592 4, 558 19. 3 32.0 7 0. 20 942 1, 217 31. 2 52. 8 0. 25 1, 378 1, 708 30. 5 47. 0 8 0.20 1. 208 1, 825 19. 0 37. 5 0.25 2, 123 4.317 14.0 27.0 9 0. 20 3, 6.933 8. 5 19. 1 0. 25 4, 10, 133 15. 5 28. 3 m 0. 20 3, 910 8, 225 6. 311. 4 0. 25 3, 872 9, 783 5. 5 10. 1 u 0. 20 1, 013 1. 487 11. 8 20. 80.25 3,038 3,642 11.2 19.8 12 0. 35 2, 467 5, 167 12. 1 21. 5 13 0. 402, 943 6, 5. 6 10. 7

It is seen from the above data that leather tanned using thepredominantly non-ionic, chromium-containing composition prepared inaccordance with the method of the present invention by reacting ligninsulfonic acid with between 25-50% sodium bichromate by weight of thelignin solids, is markedly more resistant to abrasion and waterabsorption than leather tanned with a chromiumcontaining tanningcomposition prepared by reacting only 20% sodium bichromate by weight ofthe lignin solids. This clearly shows the superior properties of leathertanned in accordance with the present invention.

To further show the superiority of leather tanned in accordance with thepresent method, various commercially available vegetable tanned soleleathers are subjected to the abrasion and water absorption describedhereinabove. The results of these tests are as follows:

- From these results it is clearly seen that leather tanned inaccordance with the present method has greatly su perior resistance toabrasion than commercially available vegetable tanned sole leather.

Moreover, it will be appreciated that leather tanned in accordance withthe present invention, i.e., in accordance with the method of Example 2,and retanned and fat liquored in accordance with the methods of Examples3 and 5, respectively, exhibits good resistance to water absorption.However, as set forth hereinabove, increased resistance to waterabsorption can be obtained by subjecting the thus-tanned leather tofurther treatment, such as with a silicone resin, e.g., Dow CorningSilicone 1109. To show the etlects of such further treatment, as well asto substantiate the fact that the superior abrasion resistance obtainedby leathers tanned in accordance with the present invention is not dueto the addition of a silicone resin, leathers produced in accordancewith the method of Example 2 and then treated with varying amounts ofthe Dow Corning Silicone 1109, i.e., -15 by weight, are subjected to theabrasion and water absorption test described hereinabove. The results ofthese tests are as follows:

Test 1.

Test 2 Abrasion Percent Water Cycles, Absorption Silicone, percent 0.105 Inches 0.5 Hrs. 2.0 Hrs.

s. o 17. 5 o. 2 13. 7 18.0 27. 3 5. 5 11. 6 5. 5 13. 8 9. 3 16.8

From this data, it is clearly seen that the waterproofness of theleathers prepared in accordance with the present method is increased bytreatment with a silicone resin and, additionally, that even where nosilicone resin is added to the leather, the abrasion and waterabsorption resistance is still greater than that of leather tanned usinga chromium-containing tanning materialprepared by reacting li-gninsulfonic acid with only 20% sodium bichromate by weight of the ligninsolids.

EXAMPLE 7 To further demonstrate the advantage of leather tanned inaccordance with the present invention, a series of actual field tests ofleather is carried out by resoling one shoe of each of three pairsandobserving the actual wear obtained in use. By this procedure, thefollowing data are obtained.

Con o! soles are good quality conventional vegetable tanned leathersoles, all control soles being at least as thick as the experimentalsoles. All experimental soles were water-proofed with Dow CorningSilicone No. 1109.

In carrying out the above tests, it is found that the experimental solesgive better traction on dry floors and pavement. Tests were concludedwhen the uppers are completely worn out, thus demonstrating that thesole should last the life of the shoe under fairly hard service.

Abrasion Percent Water Cycles, Absorption EXAMPLE 8 Silicone, percent3$;

ms. 20 Hrs Further to illustrate the practice of the present inventionwith respect to the eifect of vegetable retanning and. 7,533 m 1L4chrome retanning of leather produced in accordance 2.3g 3-; lg-g withthe present invention a series of chemical analyses 2:117 and strengthtests were carried out and indexed com- 6.383 paratively in thefollowing table, the data reported heing based on the weight of thetanned'leather as is:

Per- Per- Per- Percent Stitch Tensile Sample cent cent cent Grease HidepH Tear, Strength,

m0 Ono, Ash Substance l/In. illn.

A ma 2.10 ass 8.2a eas 4.41 can see Aplus TanolinR Retan 17.5 4.18 4.825.15 61.6 as! 851 2.138 Aplus Vegetable Betan---.--- 15.9 Lee 223 0.0464.9 3.91 ass 2,910

1 Tanned with composition produced by reaction oi 25% sodium bichromatewith llgnin solids.

' 9 The above data demonstrates that it is possible further to improvethe strength of the strong, firm leathers obtained in accordance withthe practice of this invention by proper retanning.

EXAMPLE 9 To demonstrate the analysis involved in determining the formof chrome in various reaction products of sodium bichromate and ligninsulfonic acid, a sample is prepared using sodium bichromate in theamount of 25% by weight, based on the lignin sulfonic acid content. Analiquot of this sample, in the form of an aqueous solution, is thenpassed through a cation exchange column filled with chloride-free Dowex50-X1, and a different aliquot is passed through an anion exchangecolumn, filled with sodium-free Dowex 1-X1. Th'e efiiuent from eachcolumn is analyzed for chromium and the percentage of non-ionic, anionicand cationic chromium calculated. The results of such test is set forthin the following table:

Percent Percent Percent Percent Sodium Anionic Cationic Non-ionicBichromate Chromium Chromium Chromium 7 the full intended scope of thisinvention as defined by the appended claims.

What is claimed is: 1. In the tanning of leather the method which comprises the step of subjecting the material to be tanned to an aqueoussolution of a predominantly non-ionic, chromium-containing reactionproduct obtained by chemically reacting lignin sulfonic' acid with ahexavalent chromium compound in an amount within the range of 25 toabout 50% by weight of the lignin sulfonic acid expressed as ligninsolids, to obtain a reaction product con taining a major proportion ofnon-ionic chromium.

2. In the tanning of leather according to claim 1 the further step whichcomprises retanning the thus-treated material with a vegetable tan.

3. In the tanning of leather in accordance with claim 1 the further stepwhich comprises retanning th'e thustreated material with a vegetable tanand fat liquoring the resultant leather.

4. In the tanning of leather in accordance with claim 7 1 the additionalsteps which comprise retanning the thustreated material with a vegetabletan and subsequently fat liquoring and waterproofing the resultantleather.

5. In the tanning, of leather in accordance with claim 1 the additionalsteps which comprise retanning the thustreated material with a chrometan and subsequently fat liquoring and waterproofing the resultantleather.

6. In the tanning of leather in accordance with claim 1 the further stepwhich comprises retanning the thustreated material with a chrome tan.

7. In the tanning of leather in accordance with claim 6 the further step,which comprises fat liquoring the thustreated material.

8. The process of tanning leather which comprises subjecting a pickledhide to a liquid composition comprising the reaction product obtained bychemically reacting a solution of lignin sulfonic acid prepared bymineral acid acidification of an aqueous solution of water-solublelignin solids with a solution of at least 25 and not more than about 50%by weight of sodium bichromate based on the lignin solids content, at anelevated temperature until the desired tanning is effected andthereafter adding alkali partially to neutralize the stock to a higheracid pH and thereafter washing the thus-tanned leather.

9. The method of tanning which comprises floating a pickled hide in analkali metal chloride solution and adding thereto a liquid tanningcomposition comprising the reaction product of lignin sulfonic acid anda hexavalent chromium compound in anamount within the range of 25 toabout 50% by weight of the lignin sulfonic acid expressed as ligninsolids to produce a predominantly non-ionic, chromium-containing,tanning composition, for about 2 hours at a temperature of about F.,thereafter adding an alkali metal bicarbonate solution to raise the pHof the solution to a higher acid pH and continuing the tanning until ashrink temperature of about 200 F. is obtained, and thereafter washingthe thus-treated leather.

10. A process according to claim 9 wherein the thustreated leather isretanned.

11. The method of tanning which comprises floating a. pickled hide inan. alkali metal chloride solution and adding thereto a liquid tanningcomposition comprising the reaction product of lignin sulfonic acid anda hexavalent chromium compound in an amount within the range of 25% toabout 50% by weight of the lignin sulfonic acid expressed as ligninsolids to produce a predominantly non-ionic, chromium-containing,tanning composition, for about 8 hours at a temperature of about 90? toF., thereafter adding an alkali metal bicarbonate solution partially toraise the pH of the solution to a higher acid pH and continuing thetanning until a shrink temperature of about 200 F. is obtained, andthereafter washing the thus-treated leather.

12. The method of tanning which comprises subjecting a hide to anaqueous solution of the reaction product obtained by chemically reactinga major proportion of lignin sulfonic acid as prepared by dissolving thelignin solids in water and acidifying the resultant solution, with alesser proportion of a hexavalent chromium compound, the amount ofhexavalent chromium compound being within the range of 25% to 50% byweight of the lignin solids to produce a predominantly non-ionic,chromiumcontaining reaction product, for a time suflicient to obtain ahigh leather shrink temperature and thereafter neutralizing and washingthe thus-prepared leather.

References Cited in the file of this patent UNlTED STATES PATENTS1,316,742 Robeson Sept. 23, 1919 1,553,220 Cross Sept. 8, 1925 2,117,343Merritt May 17, 1938' 2,437,044 Robinson et al. Mar. 2, 1948 2,766,098Perrin Oct. 9, 1956

1. IN THE TANNING OF LEATHER THE METHOD WHICH COMPRISES THE STEP OFSUBJECTING THE MATERIAL TO BE TANNED TO AN AQUEOUS SOLUTION OF APREDOMINANTLY NON-IONIC, CHROMIUM-CONTAINING REACTION PRODUCT OBTAINEDBY CHEMICALLY REACTING LIGNIN SULFONIC ACID WITH A HEXAVALENT CHROMIUMCOMPOUND IN AN AMOUNT WITHIN THE RANGE OF 25% TO ABOUT 50% BY WEIGHT OFTHE LIGNIN SULFONIC ACID EXPRESSED AS LIGNIN SOLIDS, TO OBTAIN AREACTION PRODUCT CONTAINING A MAJOR PROPORTION OF NON-IONIC CHROMIUM.